Regression test for SICBlockCipher position tracking (getPosition/seekTo/skip)
around counter-increment carries. getPosition() used to apply the borrow in its
counter-minus-IV subtraction as a raw byte decrement, dropping the borrow when the
decremented byte wrapped 0x00 to 0xFF - i.e. once an increment had carried across
consecutive 0xFF IV bytes - and it never subtracted IV[0], so 8-byte block ciphers
with a full-length IV reported a position offset by IV[0] * 2^56 blocks. Also
covers skip() from mid-block positions: a backward skip smaller than the intra-block
offset used to rewind to the block start instead of moving exactly |n| bytes.
Also covers the full-block IV counter-range bound: the advance since init is limited to
2^64 blocks (one carry past the low 8-byte lane is legal, reaching 2^64 blocks throws),
a backward move below the initial counter throws, and the short-IV counter-space check
fires on the processBlock() path as well as processBytes().